This article first appeared in the St. Louis Beacon, June 4, 2013: The Mississippi River’s rampage through the St. Louis region this week is a “fairly big event,” though it doesn’t come close to approaching the historic crests recorded in 1993, said hydrologist Mark Fuchs of the National Weather Service.
“We haven’t seen it this high in 18 years, and that by itself is significant,” Fuchs said.
As the Mississippi crested Monday night and Tuesday, residents of river towns faced floodwaters higher than the levels they braced for in April.
In West Alton on Tuesday afternoon, emergency sandbag levees were keeping water out of the town of 300. Officials had issued a voluntary evacuation Monday night following a levee breach near Highway 67 and Lincoln Shields Access Road. That voluntary order was expanded Tuesday afternoon to include an additional 30 residences after a second breach was reported – at the eastern most tip of the Consolidated North County Levee, according to Colene McEntee, public affairs coordinator of St. Charles County.
In Alton, the river had meandered into Riverfront Park and across the southbound lanes of U.S. 67, closing the approach to the Clark Bridge. The Argosy Casino was closed and the Con Agra Mill had implemented its flood plan, according to city officials. The Great River Road was closed, forcing motorists to use Illinois 3 to reach Grafton,where several businesses were closed.
Red pins represent current crests in the top five historically. Yellow pins represent current crests in the top ten historically. Green pins represent current crests not in the top ten historically.
Click on a pin to see more information about that location's historical crests.
The St. Louis area flooding made national headlines Tuesday, often accompanied by worrisome descriptions noting that the high water was, say, the “fourth highest level ever recorded.”
For context, we turned to data from the National Weather Service.
At the Melvin Price Locks and Dam at Alton, for example, where flood stage is 21 feet, the river appeared to have crested at 34.38 feet Tuesday night. Historically, that would make it the fifth highest. The No. 4 crest was recorded 18 years ago – on May 22, 1995, when the river reached 35.10 feet at the site. But the current crest was still 8 feet below the Great Flood of ’93, when the river crested at 42.72 feet.
In St. Louis, where the floodgates are closed and the riverfront is flooding, the river has crested at 40.52 feet, ranking it sixth on the record charts and just behind the 1844 level of 41.32. The current level is 9 feet below the highwater mark of 1993, when the river crested at 49.58. Flood stage at St. Louis is 30 feet; the city’s floodwall would be overtopped at 52 feet.
How do they measure a flood?
According to Fuchs, hydrologists measure flooding in two ways:
- level (How deep is it?)
- flow (How fast is it going?)
“Most people focus on flood levels – how high the water gets – and justifiably so, because they know what river height on the stage elevation starts affecting their businesses or their homes or the roads they use to get in and out of town,” Fuchs said.
The National Weather Service’s charts are detailed, with a hydrograph page for each gauge that details not only the crest records but also the impacts when the river reaches a certain stage. By checking the page for the St. Louis gauge, for example, you will find that Leonor K. Sullivan Boulevard begins flooding in front of the Arch at 31.7 feet, and Riverview Boulevard begins flooding at 42 feet.
“From a truly meteorological and hydrologic perspective, another thing we like to look at is a flow of record: How high the flow is, compared to what we’ve seen in the past,” Fuchs said. “When we look at it from that perspective, this event may not be one of the biggest events of all time. It may be a middle-of-the-road event. Because of all of the levees that have been built in the last 50 to 100 years, many of the flow regimes that we’re seeing now have been seen many times in the past. The water is getting up higher because of some of the levee construction.”
Flow measures how many cubic feet a second is passing through a particular point at a particular time. At 10 a.m. Tuesday morning, the flow at St. Louis was 813,000 cubic feet a second, he said.
“It takes about a half-million cubic feet per second to get above flood stage,'' Fuchs said. “If you double it, that would be the flow of record. We’re not close to that, but it’s getting up there.”
Fuchs noted that when the Mississippi reached record crests in 1993, its flow did not set records. And in January, when the Mississippi had near-record lows, the flow was still 57,000 cubic feet a second; the record low is less than 20,000 cubic feet a second.
“So it wasn’t that low, but the river stage was a lot lower because of all of the dredging. Man has affected both ends of the scale in terms of stage and in terms of the rating curve. We’re able to get lower in the river channel because of the dredging, and the river goes much higher than it used to go because of all of the levee construction. We’re making the sides of the channel taller and deeper,” he said.
What if it rains?
The forecast for rain is not good news because it will slow the recession of the river, Fuchs said.
Forecasts call for 1 to 3 inches across east central Kansas in the next week, and that rain would affect the Kansas River, which flows into the Missouri River, which has a major impact on the Mississippi in the St. Louis area. The good news is that the rainfall is expected to be spread out and not concentrated in one big event, he added.
“The river may or may not go back up, but the high waters we’re seeing on the Missouri and Mississippi will be with us for a while,” he said. “They rarely go down quickly, but it looks as though there’s enough rain in the forecast to keep them up for a while. I don’t think [the Mississippi] will get any higher than the crests we’re seeing at this moment.”
He noted that the Mississippi is expected to crest at Chester, Ill., at 42.8 feet, which would be higher than in 2011, a year that saw record flooding on the lower Mississippi.
“In Cape Girardeau, it won’t be as high as it was in 2011, and that’s simply because in 2011 the main player was the Ohio River. The closer you got to the Ohio, the greater the backwater effects,” he said. “This event has nothing to do with the Ohio River, which is low.”
